1. Field of the invention
This invention relates to a method of exposing a photo-curing type resin to light, an apparatus therefor, and a method of fabricating a printed circuit board by use of the exposing method.
2. Description of the prior art
In order that an ultraviolet-curing type ink may be applied to a surface of a printed circuit board to be hardened, the printed circuit board to which the ink has been applied is conventionally be carried by a conveyor. The printed circuit board is exposed to ultraviolet light from ultraviolet lamps during the conveyance.
The above-described method has the following drawbacks. First, radiant heat from the ultraviolet lamps increases a temperature of the ink, resulting in occurrence of air bubbles in the resin. The air bubbles sometimes reside as voids in the hardened resin.
Second, the increase in the temperature of the ink exposed to the ultraviolet light promotes or hastens hardening. This causes bridging density more than required and accordingly, excessively increases the hardness of the ink. When the hardness of the hardened ink is excessively increased, difficulty accompanies a step of grinding the hardened ink.
Third, when the ink is screen-printed on the printed circuit board, for example, an atmospheric pressure is merely applied to the ink after the screen printing. Accordingly, minute air bubbles contained in the ink during the screen printing cannot sufficiently be eliminated. The ink is thus hardened with the minute air bubbles contained therein.
Therefore, an object of the present invention is to solve the above-described drawbacks and more particularly, to provide a method of exposing a photo-curing type resin wherein the temperature of the photo-curing type resin during exposure to light can be controlled so that the voids and the excessive bridging density resulting from the temperature increase are prevented, and an apparatus therefor.
In one aspect, the present invention provides a method of exposing a photo-curing type resin which is hardened when exposed to light, comprising the step of exposing the resin to light while a base material to which the photo-curing type resin is applied is immersed in a liquid.
A liquid has a larger heat capacity and a larger specific heat than a gas. Accordingly, the liquid has a heat sinking property restraining variations in a temperature of a body in contact therewith superior to the gas. Since the light is irradiated to the photo-curing type resin while the resin is immersed in the liquid, the variations in the temperature of the resin is restrained by the heat sinking property of the liquid in contact therewith. Furthermore, liquid pressure is applied to the photo-curing type resin from around the same in the liquid reserved in the exposure tank. Consequently, even when minute air bubbles are contained in the resin as the result of the screen printing, the air bubbles can be eliminated from the resin layer by the liquid pressure applied to the resin.
In order that the above-described method may be carried out, an exposure tank reserving the liquid has a side wall which has an exposure window closed by a transparent plate, and the light from a light source disposed outside the exposure tank is irradiated through the transparent plate to the photo-curing type resin. Since the light source is prevented from coming into contact with the liquid by an insulating structure, the maintenance of the light source can be simplified.
Furthermore, a liquid circulating passage is preferably provided through which the liquid reserved in the exposure tank is circulated, and a heat exchanger is preferably provided in the liquid circulating passage for exchanging heat of the liquid so that a temperature of the liquid is maintained in a predetermined range. Consequently, the bridging density of the resin can be controlled to take a predetermined value.
Other objects, features and advantages of the present invention will become clear upon reviewing the following description of preferred embodiments thereof, made with reference to the accompanying drawings, in which:
FIG. 1 is a sectional view of an exposure tank employed in the exposing apparatus of a first embodiment in accordance with the present invention;
FIG. 2 is a view taken along line 1xe2x80x941 in FIG. 2;
FIGS. 3A and 3B are enlarged sectional views showing a through hole forming step in the fabrication of a printed circuit board in accordance with a second embodiment of the invention;
FIGS. 4A and 4B are enlarged sectional views showing a hole conductor layer forming step in the fabrication of the printed circuit board;
FIGS. 5A, 5B and 5C are enlarged sectional views showing a circuit pattern forming step in the fabrication of the printed circuit board; and
FIGS. 6A, 6B and 6C are enlarged sectional views showing a circuit pattern embedding step and a grinding step in the fabrication of the printed circuit board.